TD4 MOT Failiure, Please Help

Have a Freelander TD4 2001 with 150,000miles. Took it for MOT, Failed on the usual, brake pipes etc. The one thing it has failed on is emissions. So according to the mechanic, he changed air filter & fuel filter added some forte diesel treatment, then blasted it up the road. Emissions had lowered slightly, but still excessive. He the put a code reader which indicated an air mass fault. would this produce excessive emissions. any other ideas at what i can do.

thanks

stick a load er vedg oil innit. . . .

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